Photovoltaic devices are known in the art (e.g., see U.S. Pat. Nos. 6,784,361, 6,288,325, 6,613,603, and 6,123,824, the disclosures of which are hereby incorporated herein by reference). Amorphous silicon photovoltaic devices, for example, include a front electrode or contact. Typically, the transparent front electrode is made of a pyrolytic transparent conductive oxide (TCO) such as zinc oxide or tin oxide formed on a substrate such as a glass substrate. In many instances, the transparent front electrode is formed of a single layer using a method of chemical pyrolysis where precursors are sprayed onto the glass substrate at approximately 400 to 600 degrees C. Typical pyrolitic fluorine-doped tin oxide TCOs as front electrodes may be about 1000 nm thick, which provides for a sheet resistance (Rs) of about 15 ohms/square. To achieve high output power, a front electrode having a low sheet resistance and good ohm-contact to the cell top layer, and allowing maximum solar energy in certain desirable ranges into the absorbing semiconductor film, are desired.
Unfortunately, photovoltaic devices (e.g., solar cells) with only such conventional TCO front electrodes suffer from various problems.
First, a pyrolitic fluorine-doped tin oxide TCO about 1000 nm thick as the entire front electrode has a sheet resistance (Rs) of about 15 ohms/square which is rather high for the entire front electrode. A lower sheet resistance (and thus better conductivity) would be desired for the front electrode of a photovoltaic device. A lower sheet resistance may be achieved by increasing the thickness of such a TCO, but this will cause transmission of light through the TCO to drop thereby reducing output power of the photovoltaic device.
Second, conventional TCO front electrodes such as pyrolytic tin oxide allow a significant amount of infrared (IR) radiation to pass therethrough thereby allowing it to reach the semiconductor or absorbing layer(s) of the photovoltaic device. This IR radiation causes heat which increases the operating temperature of the photovoltaic device thereby decreasing the output power thereof.
Third, conventional TCO front electrodes such as pyrolytic tin oxide tend to reflect a significant amount of light in the region of from about 450-700 nm so that less than about 80% of useful solar energy reaches the semiconductor absorbing layer; this significant reflection of visible light is a waste of energy and leads to reduced photovoltaic module output power. Due to the TCO absorption and reflections of light which occur between the TCO (refractive index n about 1.8 to 2.0 at 550 nm) and the thin film semiconductor (n about 3.0 to 4.5), and between the TCO and the glass substrate (n about 1.5), the TCO coated glass at the front of the photovoltaic device typically allows less than 80% of the useful solar energy impinging upon the device to reach the semiconductor film which converts the light into electric energy.
Fourth, the rather high total thickness (e.g., 400 nm) of the front electrode in the case of a 1000 nm thick tin oxide TCO, leads to high fabrication costs.
Fifth, the process window for forming a zinc oxide or tin oxide TCO for a front electrode is both small and important. In this respect, even small changes in the process window can adversely affect conductivity of the TCO. When the TCO is the sole conductive layer of the front electrode, such adverse affects can be highly detrimental.
Thus, it will be appreciated that there is a need in the art for solar cell devices, and/or methods of making the same.
One aspect of certain example embodiments relates to a solar cell comprising a superstrate including aluminum-doped zinc oxide (AZO), wherein high haze is created. In certain example embodiments, the AZO may be provided at room temperature. It will be appreciated that this is advantageous, as temperatures around 200 degrees C. typically are used in connection with a-Si semiconductor processing. Certain example embodiments relate to the deposition of AZO at a temperature less than 200 degrees C., more preferably less than 100 degrees C., and more preferably at or around room temperature.
Another aspect of certain example embodiments relates to the provision of an insertion layer comprising AZO or ITO. The insertion layer may be sub-oxidized in certain example embodiments. Alternatively, or in addition, an ion beam may be used to reduce the effects associated with the ITO's crystallinity when AZO is provided thereon in certain example embodiments. In certain example embodiments, a single graded ITO layer or a single graded AZO layer may be provided in place of, or in addition to, an insertion layer.
In certain example embodiments, a method of making a front contact for a solar cell is provided. A glass substrate is provided. A dielectric coating is disposed on the glass substrate. A layer of ITO is disposed on the dielectric coating. A layer of AZO is sputter deposited on the layer of ITO, with the layer of AZO being sputter-deposited at a temperature less than 200 degrees C. The layer of AZO is etched. The substrate is baked and/or heat treated together with the dielectric coating, the layer of ITO, and the layer of AZO.
According to certain example embodiments, an insertion layer is provided between the layer of AZO and the layer of ITO. The insertion layer may comprise sub-oxidized ITO in certain example embodiments, and/or the insertion layer may have an absorption of 3-6% per 100 nm of thickness in certain example embodiments. The insertion layer may comprise sub-oxidized AZO in certain example embodiments, and/or the insertion layer may have an absorption of 2-8% (integrated over a wavelength range from 400 to 700 nm, for example) per 100 nm of thickness in certain example embodiments. According to certain example embodiments, the insertion layer may shift the 002 peak of the layer of AZO compared to a situation where no insertion layer is provided.
In certain example embodiments, a method of making a front contact for a solar cell is provided. A glass substrate is provided. A dielectric coating is sputter-deposited on the glass substrate. A layer of ITO is sputter-deposited on the dielectric coating. A layer of AZO is sputter-deposited on the layer of ITO. An insertion layer including sub-oxidized ITO or sub-oxidized AZO is sputter-deposited between the layer of AZO and the layer of ITO, with the insertion layer altering the crystalline growth of the layer of AZO compared to a situation where no insertion layer is provided.
In certain example embodiments, a method of making a front contact for a solar cell is provided. A glass substrate is provided. A dielectric coating is sputter-deposited on the glass substrate. A layer of ITO is sputter-deposited on the dielectric coating. The layer of ITO is treated with an ion beam to roughen a surface thereof, with the ion beam treating being performed at a voltage greater than 500 V. A layer of AZO is sputter-deposited on the layer of ITO. The ion beam treating alters the crystalline growth of the layer of AZO compared to a situation where no ion beam treating is performed.
Methods of making solar cells also are provided. Such methods may include connecting the front contact of certain example embodiments to an a-Si semiconductor layer and/or the like.
Certain example embodiments of this invention also relate to front contacts and/or solar cells produced using these and/or other methods.
The features, aspects, advantages, and example embodiments described herein may be combined to realize yet further embodiments.